1. Field of the Invention
The present invention pertains to radio access telecommunications networks having control nodes, and particular to the handover of a control of a connection from one control node to another control node of the radio access network.
2. Related Art and Other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a wideband code division multiple access (W-CDMA) system.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. In a phenomena know as diversity, the high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths or “legs” of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “In” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “Iub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface” or “Iu interface”. An interface between radio network controllers (e.g., between a Serving RNC [SRNC] and a Drift RNC [DRNC]) is termed the “Iur” interface.
Thus, a network such as a W-CDMA system has numerous nodes which are interconnected, for example numerous radio network controller (RNC) nodes which are connected by inter-RNC links. Typically, one radio network controller node (RNC), denominated as the serving RNC or SRNC, is assigned to control a connection. In this regard, the serving RNC or SRNC is responsible for handling the quality of the connection (e.g., termination point of the layer 2 transmission protocols such as the RLC protocol, serves as a decision-taker when the mobile should change data rate; and various administrative functions (such as logging how much the user should be charged for a particular connection, etc.). The connection is between a first party, which is a mobile party (e.g., mobile station or user equipment unit), and another party (which may either be a mobile party or a fixed party [e.g., in the PLMN]). The connection can have plural legs involving different base stations in view of the diversity capabilities mentioned above. Moreover, as explained below, the connection (with its possibly plural legs) may be routed to its SRNC over one or more inter-RNC links and through several radio network controller nodes, some of which may function as drift RNCs (DRNCs).
When a new connection is requested in a network, the system typically tries to set up the connection through the shortest path, thus making the closest network node an anchor (e.g., serving RNC or SRNC) for the connection. But as the user equipment unit (UE) involved in the connection travels geographically, one or more legs of the connection may be established through different (e.g., newly utilized) base stations to provide the diversity capability mentioned above.
If the user equipment unit (UE) travels sufficiently far, the new cell into which it ventures may be covered by a new base station for the connection. That new base station may, in turn, be controlled by another radio network controller node (i.e., a RNC which is not the SRNC for the connection). In such situation, the newly involved RNC becomes a drift RNC (DRNC) for the leg of the connection extending into the new cell. In such case, and as used herein, the term “connection” can refer to and encompass one or more of the legs of the connection in the radio access network. The serving RNC (SRNC) still controls the connection using signaling transmitted over one or more inter-RNC link(s) (e.g., over the Iur interface) which connect the SRNC and the DRNC. For example, connection combining and connection splitting operations are performed at the SRNC for the connection (see, U.S. patent application Ser. No. 09/979,866 filed Nov. 26, 1997, entitled “Multistage Diversity Handling For CDMA Telecommunications”, which is incorporated herein by reference in its entirety).
After a first DRNC is established with respect to connection involving a user equipment unit (UE), the user equipment unit (UE) may travel even further into a cell controlled by yet another RNC. As a leg for the connection with the user equipment unit (UE) is established in this further cell, this another RNC then will become a second DRNC for the connection. In such case, the second DRNC is two “hops” away from the SRNC. In other words, the second DRNC is connected by two inter-RNC links to the SNRC. Further travel of the user equipment unit (UE) may invoke yet further DRNCs in an almost telescoping manner.
If the user equipment unit (UE) travels sufficiently far, e.g., so far that a preponderance of the legs of the connection are routed to the SRNC through one or more DRNCs, consideration can be given whether the role of the SRNC should be moved over or relocated to one of the DRNCs. In such consideration, a determination is made whether the original SRNC should hand over control of the connection to a qualifying one of the DRNCs involved in the connection, e.g., a target SRNC. Such moveover is discussed, e.g., in U.S. patent application Ser. No. 09/980,013 filed Nov. 26, 1997, entitled “Diversity Handling Moveover For CDMA Telecommunications”, which is incorporated herein by reference in its entirety. See, also, U.S. patent application Ser. No. 09/035,788 filed Mar. 6, 1998, entitled “Telecommunications Inter-Exchange Congestion Control”, and U.S. patent application Ser. No. 09/035,821 filed Mar. 6, 1998, entitled “Telecommunications Inter-Exchange Measurement Transfer”, both of which are incorporated herein by reference in their entirety.
A system such as a W-CDMA system supports a wide range of users and services. The characteristics of the system can produce very heavily loaded networks. In order to utilize the networks optimally, it is of paramount importance to reduce unnecessary load in and between the control nodes (e.g., RNCs) of the network.
Utilization of a Serving RNC (SRNC) involves at least two different potential (and possibly competing) load considerations. A first load consideration is that, as the user equipment unit (UE) travels farther from the SRNC to involve an increasing number of DRNCs, there is an exponential increase in both (1) the information stored at the SRNC regarding the connection, and (2) the load over time on the Iur links (e.g., the inter-RNC links) connecting the SRNC with the DRNCs. Consequentially, to avoid overload on the Iur links, inter-RNC handover should be performed as the user equipment unit (UE) moves farther from the SRNC. However, as a second load consideration, when the responsibility for the user connection is handed over to another RNC, e.g., a target SRNC, a certain amount of signaling between and processing in the involved network nodes (e.g., the original SRNC and the target SRNC) occurs.
Some strategies for controlling inter-RNC handover have been proposed. One strategy is to presume that a constant number of RNC hops should justify the inter-RNC handover. But this strategy can lead to unnecessary handovers since a handover might be performed even if the Iur links are very lightly loaded. On the other hand, strategies that never perform inter-RNC handover will, most likely, eventually overload at least some of the Iur links
What is needed, therefore, and an object of the present invention, is a technique which judiciously controls inter-control node handovers of a radio access network by invoking various load considerations.